The woman is shopping for a bra, but she has an unusual problem. One of her breasts is much longer than the other one. The lingerie saleslady notices her customer's deformity and says, "I notice one of your breasts is longer than the other. How did that happen?"
"To be honest with you," says the shopper, "my husband can't sleep unless he has my breast in his mouth. That explains why one is so much longer than the other."
"How odd," says the clerk, "my husband does the exact same thing but my breasts are the same size."
"Yeah," says the shopper, "but I bet you don't sleep in twin beds."